High speed current interrupting electric fuses



April 20, 1965 K. v. KEELEY, SR 3,179,773

HIGH SPEED CURRENT INTERRUPTING ELECTRIC FUSES Filed Sept. 24. 1962 FIG.2. FIG. 3.

INVEN-TOR United States Patent Ofiice 3,179,773 7 HIGH SPEED CURRENTINTERRUPTING ELECTRIC FUSES Kedric V. Keeley, Sr., 1600 Wildwood Drive,Los Angeles 41, Calif.

Filed Sept. 24, 1962, Ser. No. 225,774 6 Claims. (Cl. 200-120) Myinvention is an improvement in overcurrent protection fusible link typeelectric fuses and, more particularly, in a form of construction of sucha fuse which will reliably interrupt excess current with sufficientspeed to adequately protect modern-day semiconductors and which form ofconstruction is adaptable to economic manufacture.

The phrases fusible link and fusible conductor and the word link haveone and the same meaning in this specification. The phrase effectivediameter means the diameter of a theoretical circle which circle has thesame area as an actual cross section.

The use of semiconductors has become an important part of the electronicfield and conventional fuses will often not interrupt excessive currentwith suflicient speed to prevent the excess current from damagingsemiconductors located in the circuit. Some prior art fuses of l ampererating or less will interrupt excessive current with sufiicient speed toprotect semiconductors provided the circuit voltage source issufficiently low that negligible vaporization of the link occurs withinthe fuse during the current-interrupting process. More particularly, afuse of my invention will reliably interrupt excess current even thoughthe potential of the voltage source is suflicient to vaporize the entireportion of the link and regardless of normal current rating.

A usable figure of merit of the speed of operation of acurrent-interrupting fuse is the length of time required for the fuse tointerrupt excessive current when said current is suddenly increased fromthe maximum pure D.C. continuous rated current value of the fuse to 10times this value, While the potential of the current source remains at50 volts or higher. In this figure of merit, smaller numbers are themore desirable. For the purpose of this specification, the figure ofmerit just described will be called the 10X rating. For adequateprotection of semiconductors, a fuse with a 10X rating on the order ofmilliseconds is desirable.

Another object'of my invention is to provide an improved type of fusewhich will reliably and consistently produce a x rating of 5milliseconds or less. Another object of my invention is to provide aform of economical construction of an improved fuse, the normal rating,the 10X rating and other ratings of which can be predictably controlledin the manufacturing process. Further objects and advantages of myinvention will become more apparent as the following descriptionproceeds and the features of novelty and usefulness characterizing theinvention will be pointed out in greater detail.

For a better understanding of my invention, the accompanying drawing hasbeen included in which FIG. 1 is an elevational view, partly in section,of an improved fuse embodying my invention and employing a singlefusible link.

FIG. 2 is a section on line 2-2 of FIGURE 1.

FIG. 3 is a section on line 3-3 of FIGURE 1.

Referring to FIG. 1, my invention consists of a fusible conductor 1 ofeffective diameter x, a portion 2 of conductor l and of length y beingsuspended in a gaseous atmosphere in chamber 4 and a portion 3 ofconductor 1 and of length 1 being encased in a fine textured solidinsulating material 5. The fusible conductor 1 is connected at itsdistant ends to the usual terminals 6 and 7 3,179,773 Patented Apr. 20,1965 and terminals 6 and 7 are rigidly spaced apart by the usualinsulating means 8.

Typical operation of a fusible link type, overcurrent electric fuse,when the potential of the current source remains sufiiciently high tocause an appreciable are within the fuse, is as follows:

Excessive current causes a hot spot to develop at some point along thefusible link. Metallic separation of the fusible link occurs at this hotspot, due to melting and/or evaporation, and an electric arc is formedbridging said separation. This arc conducts current while the high heatof the arc current evaporates an additional portion of the fusible linkand said arc continues to conduct current until the arc length becomestoo great for the voltage to maintain the are over the length ofevaporated fusible link or until a suitable arc path no longer exists,due to vaporization of the fusible link to a point within some form ofinsulating arc-suppression material. The speed of operation is,therefore, dependent upon the rate of fusible link evaporization and onthe length of conductor necessary to be evaporated before anarc-suppression point is reached.

In A.C. operation, some prior art fuses are able to achieve fullinterruption of overcurrent when and because the voltage of the A.C.source drops toward zero in its regular cyclic variations. Since mostA.C. sources have a frequency of 60 cycles per second, which gives ahalfcycle time of approximately 8 milliseconds, it is not feasible todepend on the cyclic zero periods of the A.C. voltage in A.C. circuitsto bring about overcurrent protection for semi-conductors. Therefore,when considering 10 ratings in this specification, no attempt is made todifferentiate between A.C. and D.C. current since thecurrent-interrupting periods under consideration are shorter thanone-half cycle of 60 c.p.s. alternating current.

Prior art (Alford, Pat. 2,159,649) has established that the relativespeed with which a fusible link disintegrates increases as the maximumcontinuous current density per cross section of link is increased.Alford further recognizes that said continuous current density can beincreased by shortening the length of the fusible link, in which caseconductive heat dissipation from the link to the more massive materialsat the ends of the link is increased and permits the link to be operatedat a greater continuous current. Alford also shows that continuous ratedcurrent density in a fusible link can be increased by using a pluralityof links of smaller cross-sectional area as compared to a single link oflarger cross-sectional area.

Prior art (Alford, Pat. 2,159,649) has also established that continuouscurrent density in a fusible link can be increased by completelysurrounding said link with a liquid or solid material capable ofconducting the heat generated by the electric current away from thelink. I have discovered, however, that if the full length of a smallfusible link is encased in insulating material, not of fiuid nature, thepoint at which the hot spot develops along said fusible link and thecurrent value at which said hot spot will develop in the fusible link isinconsistent and is difiicult to predict in manufacture because thecooling effect of the insulating material along the link depends uponthe intimacy of physical contact between the covering of insulatingmaterial and the link. Should the insulating covering material be ofgranular nature or of solid nature possessing voids near the link, whichvoids are of a dimensional size on the order of the effec tive diameterof the link or greater, the link hot spot will develop at such a pointwhere conductive cooling is the least. For this reason, difliculties areexperienced in manufacturing a link fuse of predictable rating when theentire length of said link is encased in a solid or granular materialand when the fusible conductor is of saver/a relatively smallcross-sectional area, on the order of 30 square mils or less.

I have further discovered that a link-type fuse, having a fusibleconductor of cross-sectional area on the order of 30 square mils orless, can be improved in manufacturing control of the characteristics ofthe hot spot by suspending a portion of the length of the fusibleconductor in a gaseous atmosphere, the length of said portion being notless than times greater than the largest cross-sectional dimension ofsaid fusible conductor. Under these condi tions, the effective coolingof the link by the gas is always less than the effective cooling of thelink by any contacting solid material, and the intimacy of contactbetween the gas and conductor is naturally consistent and therefore thehot spot will always develop near the center point of the portion oflength of link suspended in the gaseous atmosphere, and the currentrequired to crease a disruptive hot-spot in this construction can beaccurately and consistently predicted.

When a disruptive arc is caused to occur in a fuse link suspended in agaseous atmosphere and when an adjacent portion of the link issurrounded by and in intimate contact with a solid insulatingarc-suppression material, the length of link in the gaseous atmospherewill become vaporized following which the action of the arc is describedin prior art by conflicting statements. In Pat. 462,452, Rice statesthat metallic vapors expelled from the insulating material extinguishesthe arc while Trent, in Pat. 1,484,198, states that fusion within thecovered section of the link is merely delayed. Such a delaying actionwould, of course, lengthen the time required for complete interruptionof the current and is not compatible with fast action.

I have discovered that when the cross section of the link is maintainedat 30 square mils or less the presence of the insulating materialcontributes no detectable delay to the operating speed of the fuse andthat the tendency of the voltage source to maintain the arc inside ofthe insulating material is greatly reduced by such a small conductivecross section. I have also discovered that with a link of cross sectionof 30 square mils or less the ability of the arc suppression material tosuppress the arc is best when the largest parts of the texture of thearc suppression material are smaller than the effective diameter of theink, and if the effective diameter of the fuse link is maintained on theorder of 6 mils or less and if the length of link covered by theinsulating fine textured material is not less than 10 times the linkdiameter, reliable, consistent, and predictable extinguishment of thearc can be effected.

In further regard to normal current density in a fuse link, whilematerials are available which would permit the operation of a fuse linkat or above an incipient red heat temperature, such operation isundesirable because the link material can actually boil away at thesetemperature values which would reduce the life of the fuse, and suchhigh temperature operation would also otter certain fire hazards whenthe fuse is operated at its normal rated current.

I have also discovered that the rated continuous current density in afusible link can be made relatively high without producing red heat byusing certain materials of relatively good conductivity and operatingthe hottest spot in the link at a temperature on the order of 400 C.,which is well below incipient red heat. The better conductivity gives alow resistance value of the fusible link and consequently permits arelatively greater current to flow for operation at a given temperature.The use of materials of relatively good conductivity also enhances thecharacteristics of a fuse in that less superfluous resistance isintroduced into any circuit in which such a fuse is employed.

Alford, in Pat. 2,159,649, points out that the time required for afusible link to evaporate is dependent upon the mass of the link, theheat required to raise said link to the evaporation point, and thelatent heat of vaporization of the material. The mass of materialrequired for a fusible link of given rating is directly related to itsspecific gravity and inversely related to tie maximum permissiblecurrent density. The permissible maximum current density is, in turn,inversely dependent on the resistivity and thus said mass is directlydependent on the product of specific gravity times specific volumeresistivity of the link material. The heat required to bring a link tothe vaporization point is directly dependent on the number of degreetemperature by which the link must be raised before the boiling point isreached, and on the specifiic heat of the material, and on the latentheat of fusion. The heat required for actual vaporization is dependenton the latent heat of vaporization.

Through numerous calculations, which in turn have been confirmed bynumerous tests, I have discovered that the suitability of materials foruse as a fast-acting fusible link can be determined by the Total HeatCapacity Formula where R is the specific volume resistivity of thematerial in micro-ohm centimeters at 20 C.,

G is the specific gravity of the material at 20 C.,

H is the specific heat of the material in calories per gram at 20 C.,

T is the temperature in degree centigrade at which the materialvaporizes,

H is the latent heat of fusion of the material in calories per gram, and

I is the latent heat of vaporization of the material in calories pergram.

In this calculation, low numbers are more desirable than high numbers;for example, the calculation for gold in accordance with the aboveformula results in a figure of approximately 25,000 While for tungstenthe calculation in accordance with the formula gives a figure ofapproximately 150,000. In carefully conducted experiments, tungsten as afuse link material wa found to require several times longer to interruptcurrent overloads than was required by gold fuse links of comparablenormal current ratings. I Since it is advantageous to establish thenormal operating temperature of a fuse link on the order of 400 C.,there are certain materials which, while they would be otherwisesatisfactory as a fast-acting fuse link material, do not possesssufficient strength to provide self-support at 400 C. when suspended ina gaseous atmosphere and are, therefore, not suitable for use as fuselink materials in a fuse of my invention. Again, on conducting numeroustests, it was found that materials with a melting point near or below400 C. were not satisfactory link materials for use in a fast-actinglink fuse of my invention.

Furthermore, certain materials which were otherwise thought to besatisfactory as fuse link material for my invention were found topossess considerable tendency at 400 C. to combine with ordinary air;therefore, if these materials are used in a fuse of my invention, thegaseous atmosphere must be of a special nature as to be inert withrespect to the link material, and air must be sealed out.

I have invented an improved fuse employing the facts and discoveriesgiven above, which fuse has a l0 rating of 5 milliseconds or less andall rating of which fuse can be accurately reproduced in manufacture.

Referring to FIGURE 1, the material of which fusible conductor 1 is madehas a Total Heat Capacity Formula figure, as previously described, ofless than 50,000 and is preferably on the order of 25,000.

The effective diameter x, of fusible link 1 is not greater than 6 mils.The length y of portion 2 of fusible conductor- 1 is many times greaterthan its effective diameter x such that The length z of portion 3 offusible conductor 1 within encasing material 5 is many times greaterthan the effective diameter x of fusible conductor 1 such that Encasinginsulating material 5 is of a non-fluid nature and its texture is finerthan the effective diameter x of fusible conductor '1. The nature ofportion 2 of fusible conductor 1 and the gaseous atmosphere in chamber 4are such that they will not combine when portion 2 of the fusibleconductor 1 is operated at a temperature on the order of 400 C.

The operation of a fuse of my invention is as follows: When anovercurrent flows through fusible conductor 1, the cooling effect of theencasing insulating material on portion 3 and the cooling effect of thesupporting means of portion 2 of fusible conductor 1 confines thedevelopment of the hot spot in fusible conductor 1 near the center partof portion 2. The hot spot causes portion 2 of fusible conductor 1 toseparate and an electric arc is developed across said separation ofportion 2 of fusible conductor 1. When the power source of saidovercurrent has sufficient voltage to maintain the electric arcthroughout the evaporation of portion 2 of fusible conductor 1, suchevaporation progresses to the point where the electric arc bridgessubstantially the length of portion 2 of fusible conductor 1. At thispoint of progress, the electric arc has difficulty following thevaporization of the small fusible conductor 1 into the body of encasinginsulating material 5 and the are therefore becomes quicklyextinguished, resulting in the complete interruption of the current.

If the power source of .the overcurrent does not have sufficient voltageto maintain the arc to fully evaporate portion 2 of fusible conductor.1, then of course the arc becomes extinguished before vaporization ofportion 2 is complete and current interruption occurs in suflicient timeto provide the desired protection of the circuit.

The time required for full interruption of the current as describedabove is controllably small and if the initial overcurrent mentionedabove is 10 times more than the maximum continuous current rating of thefusible conductor 1., the time required for complete interruption of theovercurrent even though the potential of the voltage source is more than50 volts, is not more than 5 milliseconds, and greater overcurrentvalues produce even quicker interruptions.

On making tests using various values of source voltage and subjectingfuses of my invention to sudden large current over loads .1 havediscovered that as said source voltage was increased from a value ofapproximately 50 volts, the time integral of the fuse let-throughcurrent remained fairly constant and satisfactorily low until the sourcevoltage was increased above a certain value after which saidcurrent-time integral increased rapidly with increases in sourcevoltage. This point of discontinuity in the relationship between sourcevoltage and currenttime integral occurs at a relatively high value ofvoltage compared to the dimensions used on the fuses of my inventionbeing tested and it has been established, therefore, that a fuse of myinvention possesses characteristics favorable towards use in circuitminiaturization. For instance, in a fuse of my invention in which thefusible link 1 had an effective diameter x of .002" and the length z ofportion 3 encased in insulating material 5 was .15" long and the lengthy of portion 2 in the chamber 4 was .125" long, the source voltagerequired to produce the discontinuity described above was on the orderof 250 v. Thus a fuse of my invention and of this small size couldreliably be employed to protect semiconductors in circuits employingsource voltages up to 250 v.

The actual normal continuous current rating of fusible conductor 1 isdependent upon the material of which said fusible conductor 1 is made,is further dependent upon the cross-sectional area of fusible conductor1, and is further dependent upon the length of portion 2 of fusibleconductor 1.

If the length y of portion 2 of fusible conductor 1 is made at least 10times greater than the effective diameter x of fusible conductor 1, thenthe normal current rating of the resulting fuse can be easily controlledin manufacture since small percentage variations in the dimensions offusible conductor 1 and in the length of portion 2 of fusible conductor1, each causes only similarly small percentage variations in the finalnormal continuous current rating and other ratings of the fuse.

To construct a fuse of my invention with a normal current capacitygreater than can be achieved through observing the limits of fusibleconductor materials and cross-sectional areas described above, aplurality of physically and electrically identical fusible conductorscan be employed electrically in parallel physically spaced apart 1 inone encasement or as a plurality of individual single link fusesconnected externally in parallel. The operation of a fuse constructedwith a plurality of links is similar to the above-described operation ofa fuse constructed in accordance with FIG. 1, however, each of theindividual fusible conductors operates substantially independently ofthe other fusible conductors except that each carries its equal share ofcurrent. Since these fusible conductors are physically and electricallyidentical, they operate to interrupt an overcurrent substantiallysimultaneously with each other. Small variations in the electrical andphysical similarity of parallel connected fusible conductors are not ofgreat importance since, should one said fusible conductor separate atits hot spot before any others do, an additional current load would bediverted to the others and a very rapid chain reaction of disintegrationof all fusible conductors would occur. Therefore, the ratings of fusesmanufactured in accordance with FIG. 1 or with a plurality of links arevery similar excepting that fuses manufactured in accordance with aplurality of links Will possess continuous current ratings substantiallyequal to the rating of one fusible conductor multiplied by the totalnumber of fusible conductors.

As a preferred embodiment of my invention, I have constructed a fuse inaccordance with FIG. 1 in which the link 1 consists of a hard-drawn,gold wire of diameter x of 2 mils. The gaseous atmosphere in chamber 4is ordinary air and the encasing insulating covering 5 is a siliconerubber compound. The overall length of the unit is approximately /8".The length y of portion 2 of the fuse link 1 suspended in the gaseousatmosphere in chamber 4 is .125" and the length 2 of portion 3 of thefuse link 1 encased in the insulating material 5 is .2". The overalldiameter of the unit is A" and the unit fits into a fuse holder builtfor X A" tubular fuses. The long internal length of terminal 7 is usedto maintain a overall length and yet limit the link length to .325".This unit possesses a continuous direct current rating of 1.7 amperesand a 10X rating of less time than 4 milliseconds. Its full lengthresistance, operating at its maximum steady direct curlrent of 1.7amperes, is approximately .2 ohm. This design was repeatedly tested andfound to be reliable protection for semiconductors by interruptingexcessive currents from low impedance electric power sources of 300volts or less.

Prior art discussions concerning the action of fuses have indicated thatthe ability of prior art fuses to interrupt over-currents have beendependent to some extent on the current capacity of the power sourcefeeding the circuits in which said prior art fuses have been employed.Using a fuse of my invention, I have not found any such dependency. Themaximum current capacity of any source is dependent upon the totaleffective impedance of the circuit, including the internal impedance ofthe source, and on the voltage generated by the source. I have testedcircuits employing a source voltage on the order of 300 volts in which afuse of my invention was' placed and in which the only other impedanceof the circuit beside an open switch was the necessary wiring and theinternal impedance of the source. On closing the switch in a circuit ofthis nature, I have found that the instantaneous current rose to highvalues but that the voltage of the source, measured across the switchand fuse in adjacent series relationship, did not diminish appreciablyand yet my fuse successfully interrupted the current in sufficient timeto be deemed satisfactory for the protection of semiconductors.

In these tests it is therefore evident that, as the switch was beingclosed and high current began to flow, the impedance of the fuse beganto rise limiting the peak current to a value well below the maximumcapacity of the source; therefore, the use in a circuit of a highcurrent capacity source does not invalidate the usefulness of a fuse ofmy invention. In further tests, when a semiconductor with a continuouscurrent rating equal to that of the fuse was added to the circuit, thehigh current resulting when the switch Was closed was interrupted withsufficient speed that the sensitive semiconductor was not damaged.

In my preferred embodiment, I have chosen gold as a link materialbecause of its availability as finely drawn wire, its inherentinertness, its possession of adequate mechanical strength at atemperature near 400 C., its solderability for connection to theterminals, and its low figure in accordance with the Total Heat CapacityFormula previously described. Aluminum was found to be a satisfactorylink material except for the difficulty encountered in connecting to theterminals. Copper was found to be a less satisfactory link materialbecause it would deteriorate in ordinary atmosphere and so require aless common gaseous atmosphere and the necessary related hermeticsealing. Silver was found to be an unsatisfactory link material in thatit did not possess sufficient mechanical strength to remain suspendedintact in an atmosphere of ordinary air at a temperature near 400 C.(This may be due to the known tendency of silver to absorb oxygen atelevated temperatures.)

Silicon rubber was chosen as the arc-suppression, insulating materialbecause it can be easily formed over the fusible conductor and Willwithstand high temperatures and exposure to arcing.

In my preferred embodiment, greater continuous current ratings can beachieved through the use of a larger diameter fusible conductor up tothe specified limit of 6 mils or through the use of a lesser length offusible conductor in the gaseous atmosphere, down to the specified limitof diameters. Greater voltage ratings can be obtained by using longerlengths of arc suppression material.

The claims are:

1. In an improved fusible link type fuse, the improvement comprising: afusible conductor having an effective and substantially constantdiameter x along it full length of not more than 6 mils and of asufficient effective diameter to carry a normal operative current, witha portion of said fusible conductor of length y being suspended in agaseous atmospheric compartment of said fuse, and an adjacent portion ofsaid fusible conductor of length 2 being encased in a nonfluidarc-resistant insulating material within said fuse; the length y of saidfusible conductor suspended in said gaseous atmosphere being not lessthan ten times greater than the effective diameter x of said fusibleconductor and the length z of said fusible conductor encased in saidarc-resistant material being not less than ten times greater than theeffective diameter x of said fusible conductor with the texture of thearc-resistant insulating material being finer than the effectivediameter of the fusible conductor, said fusible conductor being made ofmaterial wherein the value of the following expression is less than50,000:

R is the specific volume resistivity of the material in micro-ohmcentimeters at 20 C., and has a value less than 10;

G is the specific gravity of the material at 20 C.;

H is the specific heat of the material in calories per gram at 20 C.;

T is the temperature in degrees centigrade at which the materialvaporizes;

H is the latent heat of fusion of the material in calories per gram; and

H is the latent heat of vaporization of the material in calories pergram.

2. A fuse in accordance with claim 1 wherein the melting point of thefusible conductor is greater than 400 C.

3. A fuse in accordance with claim 1 wherein the material of the fusibleconductor, suspended in the geasous atmosphere, and said gaseousatmosphere are both of such a nature that the two have no tendency tocombine when the fusible conductor is maintained at a temperature of 400C.

4. A fuse in accordance with claim 1 wherein the material of the fusibleconductor suspended in the gaseous atmosphere and said gaseousatmosphere are both of such a nature that the two have no tendency tocombine when the fusible conductor is maintained at a temperature of 400C. and wherein the melting point of the fusible conductor is greaterthan 400 C.

5. A fuse in accordance with claim 4 wherein the fusible conductor isformed of gold.

6. A fuse in accordance with claim 4 wherein the fusible conductor isformed of aluminum.

References Cited by the Examiner UNITED STATES PATENTS 856,292 6/07Phelps 200l3l 1,278,893 9/18 Eustice 20013 5 2,159,649 5/39 Alford 200-2,768,264 10/56 Jones 200-l44 2,921,250 1/60 Swain 200l20 BERNARD A.GILHEANY, Primary Examiner.

UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No.5,179,773 April 20, 1965 Kedric V. Keeley, Sr.

It is hereby certified that error appears in the above numbered patentrequiring correction and that the said Letters Patent should read ascorrected belowo Column 2, line 20, for "evaporization" readvaporization column 3, line 18, for "crease" read create column 4, line67 for "ratin read ratin s column 6 line 64 for g g 3 Q Q "curlrent"read current Signed and sealed this 8th day of March 1966.

( L) Attest:

ERNEST W. SWIDER EDWARD -J. BRENNER Attesting Officer Commissioner ofPatents UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No3,179,773 I April 20, 1965 Kedric V. Keeley Sr. 7

It is hereby certified that error appears in the above numbered patentrequiring correction and that the said Letters Patent should read ascorrected below.

Column 2 line 20 for "eva orization" read Va orization p p column 5,l1ne 18, for "crease" read create column 4, line 67 for "ratin readratin s column 6 line 64 for g g "curlrent" read current Signed andsealed this 8th clay of March 1966.

( L) Attcst:

,IERNEST W. SWIDER EDWARD -J. BRENNER Attesting Officer Commissioner ofPatents

1. IN AN IMPROVED FUSIBLE LINK TYPE FUSE, THE IMPROVEMENT COMPRISING: AFUSIBLE CONDUCTOR HAVING AN EFFECTIVE AND SUBSTANTIALLY CONSTANTDIAMETER X ALONG ITS FULL LENGTH OF NOT MORE THAN 6 MILS AND OF ASUFFICIENT EFFECTIVE DIAMETER TO CARRY A NORMAL OPERATIVE CURRENT, WITHA PORTION OF SAID FUSIBLE CONDUCTOR OF LENGTH Y BEING SUSPENDED IN AGASEOUS ATMOSPHERIC COMPARTMENT OF SAID FUSE, AND AN ADJACENT PORTION OFSAID FUSIBLE CONDUCTOR OF LENGTH Z BEING ENCASED IN A NONFLUIDARC-RESISTANT INSULATING MATERIAL WITHIN SAID FUSE; THE LENGTH Y OF SAIDFUSIBLE CONDUCTOR SUSPENDED IN SAID GASEOUS ATMOSPHERE BEING NOT LESSTHAN TEN TIMES GREATER THAN THE EFFECTIVE DIAMETER X OF SAID FUSIBLECONDUCTOR AND THE LENGTH Z OF SAID FUSIBLE CONDUCTOR ENCASED IN SAIDARC-RESISTANT MATERIAL BEING NOT LESS THAN TEN TIMES GREATER THAN THEEFFECTIVE DIAMETER X OF SAID FUSIBLE CONDUCTOR WITH THE TEXTURE OF THEARC-RESISTANT INSULATING MATERIAL BEING FINER THAN THE EFFECTIVEDIAMETER OF THE FUSIBLE CONDUCTOR, SAID FUSIBLE CONDUCTOR BEING MADE OFMATERIAL WHEREIN THE VALUE OF THE FOLLOWING EXPRESSION IS LESS THAN50,000: